We use mass spectrometry to characterize single-source precursors to the electro-optical materials lithium niobate (LiNbO3), potassium titanyl phosphate (KTiPO4, KYP), and copper indium disulfide (CulnS2, CIS). These are ternary, nonmolecular phases having complex crystal structures. New advances in chemical vapor deposition (CVD) are now allowing the growth of such complex phases from suitable single-source precursors. The primary advances are in soft precursor-delivery strategies able to introduce low-volatility precursors into the gas stoichiometries. Mass spectrometry examines the behavior and integrity of the compounds in the gas phase under conditions that crudely mimic those in the CVD reactor.